Yes, you are right. Military alone was not responsible for the 1971 debacle. It was a collective failure
But the question is that why does the Army, the country's dominant power center, always support the "wrong" side?
They always support unpopular leaders against the popular ones
Mujib was more popular than Bhutto, Army supported Bhutto
Bhutto became the most popular leader of (west)Pakistan, Army killed him
Benazir was more popular than Nawaz Sharif, Army supported (rather created) Nawaz in 1980s
Nawaz won 2/3rd majority in 1997, Army imposed martial law and exiled him
Nawaz was more popular than Imran Khan, Army supported Imran in 2018
Imran is the most popular leader in Pak ryt now, Army is supporting PDM against him
The Army always goes against the will of the majority to retain their role as country's dominant power center
The Army is not willing to concede the political power to the people of Pakistan
We have already lost half of the country because of this, and we may lose what we are left with
A lot of what is stated is because we enjoy the benefit of hindsight. Yes, the establishment has acted extra-constitutionally in the past, no denying that, however not everything is wrong/extra-constitutional or a case of the establishment going against majority. Case in point, the PTI's ouster. The establishment does what it thinks is best for Pakistan, the same as the policies of any of the political parties when in power. Do these parties always have absolute mandate when they push their policies through? No, so just like that is an imperfection, so are the calls made by the establishment.
Mujib was more popular than Bhutto, Army supported Bhutto
Army, primarily of West Pakistani origin, was cajoled to ensure West retained its leadership of the Federation of Pakistan over the East. Who cajoled the military? One should ask that question as the politicians in the West were not open/allowing MuR to form government. ZAB was hell-bent against it as were countless other Punjab based politicians. One should look up the newspaper reports from the time. I did that in microfiche format in the late 90s at the Library of Congress. Countless leaders spoke out against it and were asking the military to take action for the sake of Pakistan's survival (I was reminded of the same during the Lal Masjid Operation days when everyone was asking why the Army is not taking action only to leave the blame on the army after action was taken).
Bhutto became the most popular leader of (west)Pakistan, Army killed him
Army did not kill him. You should ask the family of Nawab Muhammad Ahmed Khan Kasuri about what transpired there. A murder was committed of this gentleman and the criminal case implicated ZAB as having been behind that case. Yes, Zia did approve his death sentence but a civilian court prosecuted his case lodged (in 1974) before Zia Ul Haq took power in 1978. So these facts should be kept in mind before assigning blame to the army. A mercy petition on the death sentence went to the President of Pakistan (Zia) and he rejected it. That was the only part that Zia played in it. No other army personnel were ever involved in either framing the case or prosecution etc.
Benazir was more popular than Nawaz Sharif, Army supported (rather created) Nawaz in 1980s
Yes, agreed that establishment supported NS because the Army high command did not trust BB as she was deemed too close to the West and anti-army then and just like it used ISI to support IK in 2017-18, similarly it tried to influence national elections. However, NS was as popular as BB (the usual Punjab vs. Sind split) so it wasn't a case of the establishment supporting a minority leader in NS.
Nawaz won 2/3rd majority in 1997, Army imposed martial law and exiled him
Army preserved its own leadership which was not being allowed to land after the then CoAS visiting SL. There were a lot of other misalignments too including Kargil, tussle with the judiciary, corruption cases etc. and the ongoing, dire situation with the economy that resulted in the establishment taking over the government. I don't condone this but also feel that in those times, the public came out thanking the military for booting NS out of office given the mess that had existed.
Imran is the most popular leader in Pak ryt now, Army is supporting PDM against him
I think it is a misinterpretation to see establishment as supporting PDM. There has been a reset till next elections. PTI took certain support for granted to stay in power. That support was withdrawn. However there is no support for PDM nor PTI currently. Imran may be the most popular leader for a certain segment of the society, but that does not translate into PTI winning absolute majority at the polls. On top of that, IK/PTI made plenty of decisions that hurt Pakistan as well and we are living through these issues now. The same goes for all involved including the establishment in the past.